History of the Related Art
The discharge of organic substances from industrial processes to the surroundings has become a serious environmental threat and methods for reducing such discharges have become more and more important. For a long time, it has been known that with the aid of certain sorbtion agents a considerable part of the contaminants in question can be trapped and subsequently rendered harmless or recovered.
Lately, interest has been focused on a group of inorganic substances, so-called zeolites as sorbtion agents, primarily since it has been possible to give them selectivity in sorbtion capacity with respect to the gaseous constituents in the contaminating agent, a selectivity which to a great degree excludes sorbtion of water vapor in spite of the fact that the injurious contaminants are separated with high efficiency. An example of how such a so-called hydrophobic zeolite can be built up is given in the Swedish Patent No. 8604873-3.
The hydrophobic zeolites eliminate a large problem with previously used sorbtion agents in the often recurring operational cases where the contaminating agent contains water vapour. The sorbtion capacity of the sorbtion agent has in such cases been appropriated to a large degree by the water vapor, reducing to a corresponding degree the sorbtion capacity for the injurious part of the contaminant. In turn, this has lead to lowered capacity of the sorbtion plant and furthermore to heavily increased operational costs for regenerating the sorbtion agent. The less tendency for water vapor sorbtion by the hydrophobic zeolites which could be provided, the more effective and operationally cheap the purification process has become.
An effective utilization of the sorbtion capacity of the hydrophobic zeolites requires, inter alia, that they are exposed in a fixed geometric structure in the contaminating agent, this structure permitting an intimate contact between zeolite and the flowing medium while at the same time permitting a large medium flow without a too high pressure drop. Such structures in the form of cellular bodies having alternatingly flat and pleated thin layers, which are caused to support each other at mutually separated places so that throughducts or gaps are formed have long been successfully used, e.g. for drying air.
A particularly advantageous embodiment is one where the starting material is a paper built up of sparcely layered mineral fibres, preferably glass fibres, which after building up the cellular structure are supplied with powder and binder filling out the gaps between the fibres and thereby sealing the layer simultaneously as the powder and binder combine to stiffen the layer so that it is given high mechanical strength. This cellular body is described in the Swedish Patent No. 8003175-0. Among the powders used has been the type of zeolite intended for absorbing moisture from air, and as a binder the precipitate occurring when waterglass is exposed to such as carbon dioxide or calcium chloride.
It has been apparently obvious to apply the same technique for achieving a cellular body for the hydrophobic zeolites, but in practice it has not been found possible to achieve acceptable results in this way. The binding agents which have been suggested have namely had hygroscopic properties, which is not a disadvantage when it is a question of bodies intended for the removal of moisture. In such a case, the hygroscopic capacity has even been desirable. However, when it is a question of fixing the hydrophobic zeolites in the cell structure, there is the requirement that the finished cellular body does not contain any hygroscopic components which would reduce the effect of the hydrophobic properties of the zeolites.